A terminal-emulation protocol that is widely used on the Internet to log on to network computers. Telnet also refers to the application that uses the Telnet protocol for users who log on from remote locations.

Microsoft Telnet> o 207.44.188.38 25
Connecting To 207.44.188.38...Could not open connection to the host, on port 25.

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The other definitions are correct, but fail to mention that telnet is very insecure, as all data (including user names and passwords) are sent across the network in plain text (unencrypted). On an untrusted/public network (like the internet) this can be quite dangerous.